1. Field of the Invention
This invention relates to a method and a device for the processing of signals representative of waves reflected or transmitted by a voluminal structure with a view to exploring and analyzing this structure.
It applies notably, though not exclusively, to the manufacture of items of equipment such as echographs, nondestructive object monitoring devices, sonars, or even radars.
2. Description of the Prior Art
Conventional equipment of this type usually uses a transmission means which transmits an incident wave into the medium to be examined and a reception means which may use all or part of the transmission means (homodyne systems) which receive the waves reflected by the structures encountered by the incident wave. A means is further provided to transform and process the signals received by the reception means and to present them in a form that can be used by the user, e.g. in the form of an image enabling the position of the obstacles generating reflections of the incident wave to be located.
The method most commonly used to obtain these results consists in using pulse waves according to a process consisting in transmitting a pulse in a given direction (shot), in detecting the return of the echoes, in measuring the amount of time lapsed between transmission and reception and in deducing the distance and therefore the position of the obstacle that generated each echo. This shooting process is then repeated for different directions, according to a predetermined sweeping law.
Once the sweeping has been performed, it is then possible to generate, e.g. on a conventional display system, images showing the obstacles detected by the echoes, and of which the positions are known.
Numerous items of equipment of this type use a so-called "sequential method" according to which the structure is examined line by line by means of a mobile beam, the exploration line being displaced after each shot.
Under these conditions, the speed of examination increases with the cross-section of the exploring beam and with the rate of the pulses. However, it so happens that the cross-section of the beam is limited by the spatial resolution required, whereas the pulse rate is limited by the time required for all the different reflected echoes to return to the probe.
For example, in order to examine a plate of aluminium to detect flaws of 1 mm in diameter, with a 3-mm resolution, the cross-section of the beam cannot really exceed 2 mm, and the pulse rate must be lower than 1,000 Hz in view of the reverberation phenomenon.
Under these conditions, the speed of surface examination cannot exceed 2 mm.times.2 mm.times.1,000=4,000 mm.sup.2 /second, i.e. 4/1,000ths of a m.sup.2, i.e. for one hour 4/1,000.times.3,600=14.4 m.sup.2. At the end of production, this speed is often too slow as it slows down production, whence the need sometimes to operate several installations simultaneously.
With numerous other applications (inspection of pipes, track rails, etc.), this limitation is even more critical.
With a view to obviating these drawbacks, it has already been proposed that there be transmitted, onto the object to be explored, a substantially flat wave, of relatively large cross-section, generated by a probe constituted by a network comprising a plurality of transmission/reception devices of small size, preferably smaller than a wave length, in order to avail of a very large radiation pattern; these transmission devices being driven simultaneously, in parallel. At reception, each transmission/reception device operates independently and therefore separately receives the waves reflected by the obstacles intercepting the beam situated in its service area. After digitization, the data supplied by these transmission devices (field of reflected waves) are stored in the memories which are read in reverse order to the order of writing.
The read signals are then applied to a device for reconstituting the field of reflected waves which comprises a plurality of transmitting devices distributed in accordance with a structure similar to that of the transmission/reception devices of the aforesaid probe. The application of the read signals to these transmitting devices is performed in correspondence with the transmission to the memory of the write signals by the transmission/reception devices.
The purpose of the reconstitution device is to reproduce the field of reflected waves in an auxiliary medium in order to reproduce an image of the object, with a resolution which depends on the wave length of the incident wave and the dimension of the probe elements.
Should the incident wave be an ultrasonic wave, the simplest solution is to form the image in an optically transparent medium and to view it by means of the Schlieren method.
However, this method is not very suitable for industrial purposes. Furthermore, it is not linear and does not enable the high frequency components to be restored.
According to another method, the image is collected on a third probe and the reading frequency is modulated so that the image of the structure is always "checked out" when the corresponding signals arrive on this probe.
It so happens that this system is complex and requires probes with a very large band. Moreover, after passing through three successive probes, the signal becomes deteriorated. Furthermore, additional difficulties occur when the transmission wave is slanted or circular.